[Xy-pic] arrow between circles

[Pol]

Hi, 

 I would like to draw a curved arrow connecting two circles, as a graphic
view of a function.
 This is my result:

\xymatrix{ & *++[o][F]{A}\ar@(dr,dl)[rr] &  & ++[o][F]{B}}

 I would like the arrow to start within the first circle and to end into the
second circle. 
 Any hints?

Thank you
---Pol